The invention relates generally to aircraft control systems and more specifically the use of a complementary filtering technique to increase the frequency bandwidth of an aircraft control system in a high vibration environment.
Advanced control techniques for aircraft rely on the use of high-gain feedback from motion sensors to achieve improved performance in terms of disturbance rejection and response compliance. A direct measure of the degree of success that can be realized in achieving these benefits is the closed-loop frequency bandwidth, which is a function of the maximum feedback gain that can be achieved. In practice, the feedback gain level is limited by either noise amplification or control limit cycle, or by a combination of both.
In high vibratory environments such as helicopters experience, excessive noise amplification occurs even for modest gain levels. The noise encountered in helicopters is associated primarily with rotor-induced vibration and, therefore, contains harmonics of the rotational frequency. The frequency content corresponding to one cycle per rotor revolution and to n cycles per revolution, where n represents the number of blades per rotor, is usually the most bothersome from a control feedback standpoint because the amplitude is large and the frequency is so low that the control actuators and control surface aerodynamics can respond. Attempts to eliminate this low-frequency noise by classical filtering techniques introduces significant lag, which aggravates the limit-cycle problem, thereby requiring reduction in the feedback gain level. For feedback control applications where low gains can provide acceptable performance, such as in stability augmentation systems, classical filtering techniques are usually adequate. For the implementation of model-following and other control concepts requiring achievement of high-gain feedback, however, the situation has been less than satisfactory, with a severe operating compromise among noise, limit cycle, and performance.
It is therefore the primary object of this invention to increase the feedback gain that can be achieved in control techniques for aircraft.
Another object of this invention is to increase the feedback gain that can be achieved in model-following aircraft control techniques.
A further objective of this invention is to increase the resistance to external disturbances in aircraft control techniques.
Still another object of this invention is to use complementary filtering in aircraft control techniques to increase the control system frequency bandwidth due to simultaneous reduction in noise amplification and control limit-cycle tendencies.
Other objects and advantages of this invention will become apparent hereinafter in the specification and drawings.